The invention relates to techniques for examining objects that are subject to small movements which interfere with the examination process, particularly when magnification is used. For example, when structures within the eye are examined at high magnification, the limiting factor often is the rapid, involuntary motion of the eye. Not only do such small eye movements interfere with detailed examination, they also frustrate the accurate focusing of magnification devices, such as a specular microscope and, therefore, make flash photography difficult. Furthermore, in treaments such as laser photocoagulation of the retina, and in laser treatments of the trabecular meshwork, eye movements can interfere with effective treatment.
A number of optical instruments such as ophthalmoscopes, biomicroscopes, or specular microscopes are used for examination of detail within the eye. For low magnification applications, such as the indirect ophthalmoscope, small eye movements, of the kind which are involuntary and cannot be controlled even by a cooperative patient, tend to be of little consequence. However, for instruments with higher magnification, such eye movements become increasingly troublesome as the magnification is increased. These small, sudden eye movements often occur at the rate of about one per second. The excursion may be only a few minutes of arc, but this can cause severe blurring of an image of corneal cells, for example. Moreover, the changes in the cornea position with such movements tends to defocus the image of interest.
Various techniques have been proposed in the past for decreasng the motion of the eye, or reducing its deleterious effect on detailed examinations. For example, one method is to contact the cornea with a so-called dipping cone objective. The outermost element of such an objective has a flat, polished glass surface which, when in contact with the cornea with a slight pressure, substantially prevents motion of the cornea along the examination axis and inhibits, although it does not completely eliminate, rotational motion of the eye. This contact can be uncomfortable for the patient, and can cause slight abrasion and consequent adverse effect on the quality of the images which can be obtained. It is also common practice to use diagnostic contact lenses, which are generally hand-held in contact with the cornea and, as a result, do not move with the eye as it rotates. Their primary function has been to allow the fundus of the eye and certain other interior regions to be viewed at low magnification, e.g. through the biomicroscope. Such lenses may reduce the eye motion somewhat due to the contact with the eye and the inertia of the contact lens. However, when the eye moves, the image seen through such a lens still moves.
Another proposal has been to use a contact lens designed to move with the eye during its small, frequent rotation, and designed to form a virtual image of the plane of interest at the center of rotation of the eye. Because such a virtual image is located at the eye's center of rotation, the image will not translate either laterally or longitudinally as the eye rotates, so long as the contact lens moves with the eye. This virtual image is then reimaged by a stationary optical system for visual examination or photography. The plane of interest may be the endothelial or epithelial cell layers of the cornea, the epithelial cell layer of the lens, regions within the depth of the lens or the vitreous, or the retina. The optical power of the contact lens, along with its thickness, determine the depth within the eye of the plane which is imaged. This proposal is described in a prior U.S. patent application of the inventor herein, filed on Apr. 14, 1980 and now issued as U.S. Pat. No. 4,410,245, dated Oct. 18, 1983, hereby incorporated by reference. While this technique helps reduce the effect of involuntary eye movements, difficulties can arise in keeping the eyelids from interfering with the positioning of the contact lens on the eye. A mild suction can be applied to keep the contact lens firmly attached to the cornea, but this can introduce other complications. The system relies on the center of rotation of the eye, which may be at different locations in various patients, and which may have a slightly different location in a given eye, depending on the nature of the eye motion. Other difficulties can arise in that the optics of the contact lens, being fixed with respect to the patient's eye, may not remain ceneterd with the optical system of the viewing device, due to patient movement.
In contrast with the known prior art proposals, this invention provides an apparatus and a method which stabilize the image of an object such as they eye, when the motions of that object are small, and when the object can be contacted. Exemplary apparatus embodying the invention includes a contact element structure comprising a contact element that contacts the eye and moves with the eye, and a lens that is fixed with respect to the contact element and forms a virtual image of a selected region of the eye. The contact element structure is supported to pivot freely about a point outside the eye, and the virtual image that it forms is stabilized in that it does not move laterally with said small eye movements. Of course the virtual image will, in general, rotate with the rotational motion of the image stabilizing system. But since the image is located at the center of rotation, the image has no lateral motion. The center of the image will have no longitudinal motion; points located away from the center of the virtual image will have a slight longitudinal motion either toward or away from the observer. When the rotational motions are small, these longitudinal motions will generally be negligible.
In one exemplary embodiment, best suited for examining the cornea, the stabilized virtual image is formed at the pivot point. When the eye moves, the contact element structure follows its motion by rotating about its pivot point, but the virtual image stays substantially stationary at the pivot point, and can be examined with an instrument such as a biomicroscope focused at the pivot point. Another exemplary embodiment is best suited for examining the fundus, and forms the stabilized virtual image at a region anterior to the center of rotation of the eye, and an examining instrument can be focused at that virtual image. The longitudinal position of the virtual image must be chosen so that it is stationary when the eye and the contact element move. In principle, the refractive error of the eye will determine the precise optical characteristics of the image stabilizing system necessary to produce a stationary image. It has been found that a lens having an adjustable position can achieve a stabilized image for a range of refractive errors.
Thus, one of the advantages of using the invention is that there is no need to place a contact lens on the eye. Another is that the center of rotation of the contact element structure is established by the invented apparatus rather than by the eye structure of the particular patient. Therefore, the virtual image is in a predetermined, stable position, and the optics remain centered with respect to the examining instrument. A further advantage is that in examining the fundus of patients with various refractive errors, image stabilization can be achieved by adjustment of the position of the lens. Numerous other advantages will become apparent from the remaining disclosure herein of illustrative embodiments of the invention.